The present invention relates to radar targets and more particularly to lightweight, buoyant, inexpensive, easily manufactured, and highly radar reflective targets, with virtually no degradation of cross section caused by lack of flatness and orthogonality of the reflecting surfaces; and, to an efficient radar augmentation device.
Corner reflectors have long been established as effective radar targets for navigation and calibration because they can provide strong radar echoes over large angles. Their inherent simplicity of design makes them uniquely attractive for deployment in unattended and inclement environments. FIGS. 1a, 1b and 1c show geometric configurations of some commonly used reflectors. With a triangular corner reflector as in FIG. 1a, the radar cross section or strength of the radar return (RCS) is expressed as ##EQU1## where a=the length of the corner, and .lambda.=radar wavelength. In the circular corner reflector of FIG. 1b, ##EQU2## and with the square corner reflector of FIG. 1c, ##EQU3##
The corner reflector derives its large radar reflectivity from the fact that every ray of energy incident on it is subjected to one or more reflections and is thus redirected to the source as shown in FIG. 2. Viewing FIG. 2: from optics, the angle of incidence=the angle of reflection, i.e., .phi..sub.i =.phi..sub.r ; from geometry, a line crossing two parallel lines form equal angles, i.e., .phi..sub.r =.phi..sub.i, and from optics, .phi..sub.i =.phi..sub.r ; so .phi..sub.i .phi..sub.r =.phi..sub.i =.phi..sub.r, or .phi..sub.i =.phi..sub.r, and the incident and reflective radar waves are parallel.
The critical factor in the fabrication of high quality reflectors is the requirement that the reflector surfaces be precisely flat and orthogonal. The general rule to be followed is that compliance to flatness and orthogonality should be less than one quarter of the radar wavelength. For typical radars, this is on the order of 1/4 inch. Departure from these specifications causes drastic degradations in the reflected signal strength and uniformity of angular response.
The problems of fabricating efficient reflectors become particularly acute for sizes in excess of two feet at the short wavelengths of modern radars. Experience has shown that the inherent warp of rolled metal stock used for fabrication and the distortions induced in the welding process cause significant degradation. Solutions to these problems require the use of metals that have been machined flat or necessitate costly fastening processes that allow alignment of the component plates. The cost of a triangular reflector four feet along the corner, for example, typically ranges between $500 and $750.